1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
An electrophotographic apparatus or an electrostatic recording apparatus (hereinafter, an image forming apparatus) such as a copying machine, a printer, or a facsimile includes a developing assembly for visualizing an electrostatic latent image using a non-magnetic single-component toner. Conventionally, a developing assembly which includes a developing roller as a developer bearing member for bearing and conveying toner and a supply roller disposed around the developing roller and serving as a developer supply member for supplying toner to the developing roller is known. In this developing assembly, toner is supplied to the developing roller while being triboelectrically charged by mechanical rubbing between the supply roller and the developing roller. The supplied toner, of which the thickness of a toner layer on the developing roller is regulated to a predetermined amount by a developer regulating member, is conveyed to a developing zone near a photosensitive drum, which is an electrostatic latent image bearing member, and the electrostatic latent image is visualized as a toner image.
Toner which remains on the developing roller without being used for development in the developing zone (hereinafter referred to as a “residual developing toner”) is scraped off the developing roller by mechanical rubbing between the supply roller and the developing roller in a contact region contacting the supply roller. Simultaneously with this, toner is supplied from the supply roller to the developing roller. On the other hand, the scraped toner is mixed with toner present inside and near the supply roller.
Conventionally, depending on a printing pattern during an image formation period in such a developing assembly, a phenomenon in which a halftone density immediately after a background portion is different from a halftone density (hereinafter referred to as a “development ghost”) immediately after solid print may occur. The development ghost occurs due to a difference in toner charge amount which results from a difference in printing pattern and is likely to occur when the supply roller has low scraping performance.
In particular, in a developing assembly in which the respective surfaces of a developing roller and a supply roller rotate in the same direction in the contact region therebetween (called a “co-rotating developing assembly” below) as in Japanese Patent Application Publication No. 2013-228584, there have been cases where development ghost has occurred in marked fashion due to the weak mechanical stripping action of the supply roller.
In this case, in order to reduce image density non-uniformity in the leading end portion of the image, there are examples in which the biases applied respectively to the developing roller and the supply roller differ between the front half portion and the latter half portion of the development of the first page of paper after a print preparation operation of the developing roller, as disclosed in Japanese Patent No. 5062183. The problem in Japanese Patent No. 5062183 is that in the first page of paper after the print preparation operation, in particular, the density is high during one revolution of the developing roller from the leading end of the image, and becomes weaker thereafter.